Generally, various processes, e.g., a film forming process, an etching process, an oxidation/diffusion process, an annealing process and a modification process, are performed on a semiconductor wafer serving as an object to be processed in a manufacturing process of a semiconductor integrated circuit. For example, in the film forming process, a polysilicon film, a SiO2 film, a W (tungsten) film, a WSi (tungsten silicide) film, a Ti (titanium) film, a TiN (titanium nitride) film, a TiSi (titanium silicide) film and the like are deposited on a surface of the wafer. A heat treatment apparatus performing such a film forming process is disclosed in, for example, Japanese Patent Laid-open Application Nos. H9-237763 and 2001-23966.
Here, a conventional general single-wafer heat treatment apparatus will be described with reference to FIG. 7. FIG. 7 shows a configuration of a conventional general single-wafer heat treatment apparatus. The heat treatment apparatus includes an evacuatable processing chamber 2, wherein a shower head 4 for introducing various necessary gases into the processing chamber 2 is installed in a ceiling portion thereof. Further, inside the processing chamber 2, a placement table 8 of a thin plate shape is supported on a cylindrical support column 6 which is extended upward from a bottom portion of the chamber, and a semiconductor wafer W is mounted on an upper surface of the placement table 8. A plurality of heating lamps 10 is installed under the processing chamber 2. Heat rays emitted from the heating lamps 10 are introduced into the chamber through a transmission window 12 provided in the bottom portion of the chamber, whereby the wafer W as well as the placement table 8 is heated.
A N2 gas serving as a purge gas is supplied into a space under the placement table 8 by a purge gas feeding unit 14 to thereby prevent a processing gas from entering into the space under the placement table 8. Accordingly, an unnecessary film is prevented from being adhered to an upper surface of the transmission window 12 or a lower surface of the placement table 8. If an unnecessary film is adhered to the transmission window 12, a transmittance of the heat rays becomes decreased to thereby deteriorate a heating efficiency. Further, if an unnecessary film is unevenly adhered to the lower surface of the placement table 8, a light absorptance becomes nonuniform and a temperature distribution is developed on the placement table 8 to thereby decrease an in-surface uniformity of the wafer's temperature. In order to overcome such a drawback, it is very important to supply the above-mentioned purge gas.
For example, in the aforementioned film forming process, if a diffusion rate of a processing gas used for film forming is not so high, the processing gas can be sufficiently prevented from entering into the space under the placement table 8. However, in case of using a processing gas having a very high diffusion rate, e.g., a silane gas, even though the purge gas is supplied into the space under the placement table 8, the processing gas enters into the space under the placement table 8 from a small gap, whereby an unnecessary film can be adhered to the lower surface of the placement table 8 or the upper surface of the transmission window 12.
In order to avoid such a phenomenon, it can be considered to substantially increase a flow rate of the purge gas. But, if so, a pressure inside the space under the placement table 8 is also increased. Since the increased pressure is exerted on the whole lower surface of the placement table 8, it is possible that the placement table 8 is raised to thereby generate a misalignment of the placement table. Further, in case that the placement table 8 is strongly fixed to the support column 6, the placement table 8 itself can be destroyed. Besides, since a flow speed of the purge gas jetting outwardly through the gap between the upper surface of the support column 6 and the lower surface of the placement table 8 becomes considerably greater, a circumferential portion of the placement table 8 is partially cooled to thereby deteriorate the in-surface uniformity of the temperature of the wafer W. Accordingly, it is not allowable to sharply increase the flow rate of the purge gas.